Digital Subscriber Line (DSL) systems are commonly used as one of the methods of transmitting data when there is a wired connection, such as a twisted wired pair, between a transmitter and a receiver. Modems which operate in an Asymmetric DSL (ADSL) mode typically use a multitone signaling technique known as discrete multitone (DMT) signaling, and a DMT signal may be considered to be composed of a large number of sinusoidal signals. Each sinusoidal signal has a relatively small varying amplitude and phase, but the DMT signal which is the sum of the sinusoidal signals typically has a large dynamic range. In other words, even though the average signal voltage amplitude is low, the DMT signal contains high and infrequent voltage peaks. To avoid signal distortion, the peaks must be transmitted and received accurately.
Typical ADSL modems utilize a relatively high voltage power supply in order to transmit the DMT signal without distortion. Accordingly, line drivers of typical modems use power inefficiently, since most of the time the signal being sent has a low voltage amplitude. The resultant low efficiency of the line drivers means that the power consumed is large. Thus, when many modems must be deployed in a single location, as is the case, for example, in the central office of a regional telephone company, the heat generated because of the low modem efficiency limits the number of modems that can be installed in a given space.
For efficient operation, all modems need to have good impedance matching at the output of the modem. Typically, a resistor in series with the output of an operational amplifier, which acts as a line driver signal amplifier, is used to define the output impedance, which is set equal to the load impedance to achieve good impedance matching. In this situation the resistor dissipates half the power delivered by the amplifier, and the load only receives half of the amplifier's voltage output.
Methods are known in the art to decrease the power consumption of line drivers for signals with a high dynamic range. In an article entitled "Line Driver Economically Synthesizes Impedance," by Koren, in the Jan. 6, 1994, issue of Electronic Design News, which is incorporated herein by reference, there is a description of a method of synthesizing the output impedance so as to reduce the value of the resistor in series with the output of an operational amplifier. The method utilizes two feedback loops, both using resistors. In order for the driver to work correctly, all resistors in the system must have very close tolerances and must be carefully matched.